maybe the following happened.
plot1 = Plot[Sin[x], {x, 0, 2 \[Pi]}, DisplayFunction -> Identity];
Show[plot1, plot2, DisplayFunction -> $DisplayFunction];
sometimes this can happen if unknown code does it and doesn't undo it
(or is stopped before it does).
you might even be "auto loading" a package you once installed which
does it to you every time you start mathematica. or maybe it's stored
as a preference somewhere in some init.m less likely is
$DisplayFunction isn't working - but it's possible.
if it persists across sessions (after quit / restart) you may have
install problems and consider re-installing